Abstract: Intracellular [H +] (pHi) and [Ca++] changes are important in the activation of sea urchin gametes. The goal of this work is to describe the mechanisms of these ionic movements at the level of isolated membranes. The Na+ dependent increase of pHi responsible for initiating sperm motility will be studied at three levels: the intact sperm, isolated sperm heads and tails, and membrane preparations derived from them. Methods will be developed to monitor membrane potential and pHi in these preparations. The possible differences in ion transport properties in sperm heads and tails correlating with their functions (acrosome reaction and motility respectively) will be examined. Plasma membranes will be prepared from eggs and zygotes and the Na+ dependent H + transport activity studied. Efforts will be made to identify the factors involved in activating this activity after fertilization. The acidic granules appearing in zygotes concomitant with the rise of pHi will be isolated and characterized, and their function elucidated. Research is also proposed to prepare intact acrosomal granules and their membranes. These membranes will be characterized, their ion transport properties studied and efforts made to use them as an in vitro system for studying membrane fusion occurs during the acrosome reaction and fertilization.